In recent years, due to rapid technological progress in the electronics field, miniaturization and weight reduction of electronic devices is progressing. As a result, portable and cordless electronic devices have been developed, and it has been desired in a secondary battery serving as a drive source of such an electronic device to achieve miniaturization, weight reduction, and high energy density. In order to meet the requirements, lithium secondary batteries having high energy density have been developed. These days, ultrathin and lightweight secondary batteries using an aluminum laminate film for a casing member have been developed and further commercialized.
In secondary batteries using such an aluminum laminate casing member, the battery thickness changes due to volumetric expansion and contraction of an electrode owing to charge-and-discharge operations. Due to this, there is the problem that the battery may twist, or a distance between electrodes may widen to increase resistance of the battery, so that battery characteristics are lowered.
There have been developed batteries using, in a negative electrode active material, lithium titanate as a material capable of avoiding such a problem. The volume of lithium titanate hardly changes due to charge-and-discharge operations, so that a change in battery thickness is extremely small. The above problem is solved by using this compound as a negative electrode active material.